SQL update

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I know how to update a field in one file with data from another file using
SQL.  But, I need to update more than one field now.  I vaguely recall
seeing a "trick" like that somewhere but don't recall where.  Anyone know
how to accomplish this?

What I tried is the following:

update file1 a set field1, field2 = (select field3, field4 from file2 b
where a.key1 = b.key2) where exists (select * from file2 b where a.key1 =
b.key2)

Dave